Ship&#39;s stern and propeller arrangement



Dec. 17, 1968 P. c. PlEN SHIPS STERN AND PROPELLER ARRANGEMENT FiledJan. 31, 1 967 2 Sheets-Sheet 1 FIG.

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'INVENTOR PAO CH/ PIE/V Dec. 17, 1968 RIC. PIEN 3,416,480

SHIPS STERN AND PROPELLER ARRANGEMENT Filed Jan. 31, 1967 v 2Sheets-Sheet 2 INVENTOR P40 CH/ PIE/V ATTORNEY United States Patent3,416,480 SHIPS STERN AND PROPELLER v ARRANGEMENT Pao C. Pien,Montgomery County, Md., assignor to the United States of America asrepresented by the Secretary of the Navy Filed Jan. 31, 1967, Ser. No.613,043 4 Claims. (Cl. 11457) ABSTRACT OF THE DISCLOSURE A ships sternand propeller arrangement having twin laterally spaced and overlappingpropellers mounted symmetrically with respect to the centrallongitudinal plane close to the main hull on shafts extending from sterntube bossings which are smoothly faired into the sterm form or which arecarried on short struts attached to the stern. This arrangement utilizesthe energy of the water in the viscous wake-belt which normally followsthe ship due to skin friction, yet avoids the cavitation and vibrationproblems normally associated with single-screw ship configurations.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

Background of the invention This invention relates generally to shipshull form and screw propeller arrangements and more particularly to twinlaterally spaced propellers with overlapping fields. It recognizes thelong overlooked fact that the energy of the water normally following theship may be captured without the attendant problems of cavitation andvibration by using twin propellers properly phased and mounted close tothe ships bull in the viscous wake-belt. This viscous wake-belt may bedescribed as the water close to the ships hull which has some forwardmotion due to skin friction between the water and the moving hull.

The prior art discloses many attempts by experts in this field toincrease the propulsion efficiency and at the same time to eliminate thecavitation and the vibration problems, but no significant advances havebeen made to date. While many have recognized propeller limits of loadand cavitation, they have generally solved this problem by usingmultiple propellers to decrease propeller loading and have placed suchpropellers away from the hull in the clear undisturbed water to avoidcavitation and vibration problems. These arrangements necessitated theuse of long exposed shafts and bearing struts which induced eddycurrents and considerable resistance. The inventers of the prior artfailed to recognize that they could obtain increased efficiency bymounting the propellers close to the hull and thereby capture the energyin the viscous wake-belt created by skin friction and yet minimize oreliminate cavitation and vibration problems by using two overlappingpropellers properly phased.

In particular, Lundborg (Patent No. 384,047) saw advantages in twinpropellers, but he mounted them away from the hull in clear water.Browne (Patent No. 162,- 619) also recognizing the need for twin screwsto reduce propeller loading, chose concentric counter-rotatingpropellers for efiiciency, but did not solve the problems of cavitationand vibration.

Summary From the above exemplications it can be seen that the prior artteaches away from the present invention. The instant invention takesadvantage of the energy of the water in the viscous wake-belt withoutthe attendant problems of cavitation and hull vibration which had notbeen previously done in spite of the extensive work of expert shipdesigners working in this art. Firstly, by placing propellers close tothe hull of the ship, these propellers will be working in the viscouswake-belt from which energy can be recovered. Secondly, the use of twopropellers overlapped and properly phased eliminates the need for longexposed shafts and struts with their attendant appendage resistance.Thirdly, by overlapping the propellers, the action of one propellerevens out the inflow to the other and thus evens out propeller loadingto reduce vibration and cavitation where heretofore, propellersoperating the wake-belt, which normally has great velocity variations,created excess vibration and cavitation. Experts in the field did notover many years recognize the unobvious solution to the problem.

The general purpose of this invention is to increase propulsionefficiency, increase speed and endurance of a ship and decrease fuelconsumption. To attain these ends, the present invention contemplatesplacing two propellers in an overlapping, laterally spaced relationshipsymmetrical with respect to the central longitudinal plane of the ship,and in close proximity to the hull of a single screw type ship. Thepropeller shafts may be carried by bossings which are smoothly fairedinto the stern form, or by short struts attached to the stern formhaving lengths of approximately 30%55% of the propeller diameter. Thepropellers may rotate in the same plane, in which case they must bephased and counter-rotating, they may be in offset longitudinally spacedclosely adjacent planes, or the blades may be raked forward in onepropeller and aft in the other propeller in which case they may rotatein either the same or opposite direction.

Accordingly a primary object of this invention is to capture energyexisting in the water of the viscous wakebelt normally following theship.

Another object is to decrease or eliminate cavitation and vibrationproblems.

Still another object is to eliminate appendage resistance.

An overall object of this invention is to increase fuel economy, speed,and endurance.

Other objects and features of the invention will become apparent tothose skilled in the art as the disclosure is made in the followingdetailed description of the preferred embodiment of the invention asillustrated in the accompanying sheets of drawings in which:

Brief description 0 the drawings FIG. 1 is a perspective quartering viewof a single screw cruiser type stern form;

FIG. 2 is a perspective quarting-view showing a transom type stern form;

FIG. 3 is a perspective straight-on aft view more clearly showing thesingle screw form and overlapping propellers close to the hull; and

FIG. 4 is a water line plan view of a vessel moving in water showingdiagrammatically the viscous wake-belt.

Description of the preferred embodiment Referring now to the drawings,wherein like reference characters designate like or corresponding partsthroughout the several views, there is shown in FIG. 1 the after portionof a cruiser type stern hull form designated generally as 10. Thewater-line is generally designated by a dashed line 12 separating thefree-board portion 14 from the submerged bottom portion 16 of the hullform. The extreme after end of the stern form of the cruiser type hullis convexly curved as shown generally at 18. FIG. 3 more clearly showsthat there is no break in the smooth continuity of the lines and thatthere is no discrete transom.

At the extreme after end of the hull form 16 below the water line and onthe central axis of the hull, there is a rudder 20 supported by a rudderstock (not shown). The space between the skeg or vertical stern frame 26and the rudder 20 forms a stern aperture 27.

On either side of the central vertical plane of the hull there is astern tube bossing 29 equally laterally spaced and smoothly faired intothe lower hull form of skeg or vertical stern frame 26. Within theaperture 27 and on shafts extending through the stern tube bossings 29are propellers 25. As shown in FIG. 1, but more clearly in FIG. 3, thesepropellers are overlapped. Although shown as physically overlapped it isto be understood that only the propeller induced velocity field need beoverlapped to provide the advantages of this invention. As can be seenin FIG. 1, when the propellers are rotating, the tips pass in closeproximity to the main hull form, where the viscous wake-belt exists.Alternatively, these propellers may be offset longitudinally spacedplanes, in which case they rotate in the same direction or in oppositedirections. Also, the propellers may be in the same transverse plane andthe blades of one raked forwardly and the blades of the other rakedaftwardly to avoid blade contact, and then they may rotate in eitherdirection. It is obvious that when the propellers are overlapped and inthe same transverse plane and When the blades are not raked, thepropellers must be counter-rotating to avoid blade damage.

FIG. 2 shows an alternative embodiment of this invention wherein thestern form is of the transom type having a discrete transom 30, andwhere the after bottom portion 16 is flatter from beam to beam and cantsforwardly and downwardly to the keel line well forward. The keel line isextended horizontally and aftwardly to terminate in a stern tube bossingwhich is attached to the main hull form by a thin web or skeg 26 or thepropeller shafts may be attached to the hull by struts (not shown).Either a single rudder in the central plane or two rudders, one behindeach propeller, may be provided. This transom type ships stern form andpropeller arrangement provides the same advantages as the preferredembodiment by placing the propellers close to the hull in the viscouswake-belt. The princpal is also equally applicable to submarine typehulls.

Description of operation FIG. 4 shows in diagrammatical form, theevolution of the viscous wake-belt 32 at and below the water line of aship traveling forwardly through the water. This viscous wake-beltdevelops because of the skin friction between the hull and the displacedwater. When the ship is moving forward, the water adjacent the skinclings and is thereby carried forward with the ship. It can beunderstood that there is less relative motion between the water in theviscous wake-belt and the ship than there is between the undisturbedclear water and the ship. As a result, considerable energy is impartedto this water and it is the intent of this invention to capture thisenergy and thereby increase efficiency of the hull and propulsionsystem. It is also known that the water in the viscous wake-belt has ahigh velocity variation which in the past often created severecavitation and hull vibration. The use of two propellers eitherphysically overlapped or having their fields overlapped, etfectvelyreduces the pulsations which occur when a propeller blade moved throughthe velocity gradient existing in the viscous wake-belt because eachpropeller evens out the inflow to the other. And obviously, the use oftwo propellers reduces propeller loading and thereby cavitation, whichtherefor allows more power to be put into the propulsion system.

Various modifications are contemplated and may obviously be resorted toby those skilled in the art without departing from the spirit and scopeof the invention, as hereinafter defined by the appended claims, as onlythe preferred embodiments thereof have been disclosed.

What is claimed is:

1. A ships stern and propeller arrangement comprismg:

a rudder;

a stern form having a vertical stern frame spaced from said rudder toform a stern aperture;

a pair of propellers; and

a pair of propeller shafts having the propellers mounted thereon,extending aftward out of said stern form into the stern aperture, saidshafts being laterally spaced equidistantly from the longitudinal centerplane of the ship so as to dispose said propellers in the ships viscouswake belt, and the spacing and diameter of the propellers being sorelated as to dispose each propeller in the induced velocity field ofthe other propeller to achieve a uniform inflow to each propeller.

2. The ships stern and propeller arrangement of claim 1, wherein thespacing of the propellers is from 100% of the propeller diameter.

3. The ships stern and propeller arrangement of claim 1 wherein thepropellers rotate in the same plane.

4. The ships stern and propeller arrangement of claim 1, wherein thepropellers rotate in longitudinally spaced planes.

References Cited UNITED STATES PATENTS 1,302,982 5/1919 Shonnard --371,628,837 5/1927 Gebers 115-37 2,283,291 5/1942 Selden 114-57 2,371,0133/1945 Wosenitz 11537 FOREIGN PATENTS 11,676 1884 Great Britain.

ANDREW H. FARRELL, Primary Examiner.

US. Cl. X.R.

